One of the most important problems in electronic surveillance involves the recognition and identification of a signal in a multi-signal environment. In the past it has been common to use so-called compressive receivers for rapidly sweeping out a given band of frequencies to determine the presence or existance of a signal and its frequency. However, the mere identification of the presence of a signal and its frequency is oftentimes insufficient to obtain the identity of the signal. It is therefore important to identify some other signal parameter as a key to the source of the signal. In this invention the parameter selected is modulation.
With the present day compressive receivers in which the incoming signal is heterodyned with a fast sweeping local oscillator signal, the sampling rate is too slow, e.g. below the Nyquist rate, to be able to completely demodulate the output of the compressive receiver. This means that the sampling time for any given signal is insufficient to permit complete demodulation.
It will, of course, be appreciated that since compressive receivers sweep a band of frequencies rapidly, parallel processing of many channels is possible with the result that copious statistics from all signals in the particular band of interest are available. This means that the high frequency band can be divided into 10,000 different frequency recognition channels. Even with the rapid scan now available practically all signals of interest are insufficiently sampled for purposes of demodulation. Although this lack of valid sampling prevents demodulation, it is a finding of this invention that information about the type of modulation is nonetheless present.
It will be appreciated that in present day compressive receiver, once a signal of interest or possible interest has been found, the incoming signal is switched to a separate receiver for demodulation. However, typically there may be as many as 3,000 to 5,000 signals present or active at any one time, most of which are not of interest. The problem then becomes how to determine the modulation format of the signals while still utilizing a single compressive receiver. It should be noted that the usual technique for sorting signals involves completely parallel receiver channels. As will be seen, the subject technique utilizes only one receiver with parallel circuits for recognizing the type of modulation on the signal. Thus, the subject system may be easily retrofitted to any one of a number of conventional compressive receivers.
In general, the compressive receiver is one which employs a variable frequency oscillator. This oscillator is swept such that its signal, when mixed with an incoming signal (should one be present) produces a linear FM signal. The linear FM signal is coupled to a dispersive delay line which time compresses the linear FM signal. When the output of the dispersive delay line is displayed as a function of time, the position of the compressed pulse on the time axis correlates to the frequency of the incoming signal. This type receiver provides for extremely fast sampling of all the frequencies within, for instance, a 30 MHz band. In the usual case the "revisit" time, that is the time between samples at a given frequency, is on the order of 40 milliseconds.
However, with an incoming signal being sampled only once every 40 milliseconds, the sampling is at less than the Nyquist rate for most signals of interest and it is therefore impossible to completely demodulate the incoming signal.
In view of the foregoing, it is a finding of this invention that there is in fact enough information in the compressive receiver output to recognize the type of modulation.
It is a further finding of this invention that the type of modulation of an incoming signal can be ascertained by the utilization of so-called "histograms." A histogram as defined herein is a graph or correlation of a "designated condition" (bin condition) versus the number of occurrences of the "designated condition" (bin condition) in a given time period called the "data collection interval." This data collection interval in general is made sufficiently long to allow a relatively large number of modulation state changes to have occurred.
As will be seen, the "designated condition" could be the number of "consecutive" 1's or 0's in a transmission, where a 1 or a 0 would be the occurrence of a pulse above or below a given threshold. On-off keying (OOK) can thus be recognized since characteristically these transmissions have recognizable numbers of "consecutive" marks or spaces. The "designated condition" could also be the frequency of the incoming signal or its amplitude.
An example is the recognition that a morse code signal is being sent. In this case, one "designated condition" might be the occurrence of three consecutive 1's versus the number of times that three consecutive 1's condition occurs in a 15 second "data collection interval." Thus, in a 15 second period there might be 14 occurrences of three consecutive 1's and this might be characteristic of a typical morse code signal. In fact, the subject system can distinguish between many types of on-off keying (OOK) signals such as hand morse, machine morse and anomolous morse, which is a morse in which bursts of characters are sent followed by long intervals of dead time.
In addition to distinguishing between different types of OOK signals, the subject system can also distinguish between other types of signals such as frequency shift keying (FSK), phase shift keying (PSK), AM, SSB, and multitone modulation. Moreover, it is possible to detect the presence of voice communication.
Thus, histograms need not be confined to whether there are consecutive 1's or 0's in the transmission, but may also include the frequency distribution of the incoming signal versus the number of times that a given frequency exists, as well as an amplitude distribution charting in which the number of occurrences of each particular amplitude are recorded over a given data collection interval. In short, it is also possible to measure for how many samples a signal is at a given amplitude or for how many samples a signal is at a given frequency.
As another example, if the "designated condition" is frequency, the incoming signal may, for instance, be an FSK signal at 27000.250 KHz or 27000.650 KHz depending on the modulation state. Generating a frequency histogram would result in a graph of the 27000.250 KHz and the 27000.650 KHz conditions versus the number of times the incoming signal was at either 27000.250 KHz or 27000.650 KHz during 15 second "data collection interval." This is unlike spectrum analysis because the ordinate is not amplitude but rather the number of times that a given frequency occurs during a given "data collection interval."
The result of generating these types of histograms is a diagram, correlation, or pattern which is characteristic of the modulation type, such that FSK, PSK, hand morse, machine morse, anomolous morse, SSB, multitones, and AM can be distinguished one from the other. In general, the histogram is generated at a histogram generator and the output of the histogram generator is coupled to a processing unit for the recognition of the particular type of modulation of interest. Once such a signal exists an alarm may be initiated and a conventional receiver can be activated to fully demodulate the incoming signal.
For instance, if it is desirable to choose only FSK signals, all other signals from the compressive receiver can be ignored and the signals of interest can then be shunted to a conventional FSK receiver for demodulation, thereby precluding the necessity of shunting the many signals that are present to a variety of different conventional receivers for demodulation. By utilization of this modulation sorting technique the number of conventional receivers may be reduced with a concomitant reduction in time to signal acquisition.
The subject histograms therefore provide a new type of graphing correlation and display technique for portraying the type of signal which is being received. It also provides a unique method of identifying signals from a compressive receiver by specifying the modulation-related characteristics which are sought.
In order to supply the requisite signals for frequency histogram production, a specialized device called a "center finder pulse" generator is utilized which in essence tracks the envelope of the signals from the compressive receiver, finds the peak of the envelope and produces a pulse at this time. By generating the center finder pulse at a particular time corresponding to the particular frequency sweep of the compressive receiver, its location on the abscissa specifies the frequency of the incoming signal (including modulation). One type center finder pulse generator described hereinafter results in frequency histograms having frequency bins as narrow as 55 Hz. This permits recognition of FSK modulation.
Moreover, for the 1's, 0's histograms a specialized 0's and 1's histogram data generator is provided, which includes a unique combination of a conventional thresholding circuit and a latched counter.
It is therefore an object of this invention to provide a method and apparatus for signal recognition in which histograms are utilized in characterizing the type of modulation on the signal.
It is a further object of this invention to provide a method and apparatus for identifying the modulation type of signals which are insufficiently sampled for complete demodulation.
It is another object of this invention to provide a method and apparatus for identifying the signals from a compressive receiver.
It is yet another object of this invention to provide a system for identifying incoming signals utilizing a single receiver and parallel histogram type modulation sorting apparatus.
It is a yet still further object of this invention to provide a signal recognition system involving the use of compressive receivers and a center finder pulse generator.
It is another object of this invention to provide a unique consecutive 1's and 0's histogram generator.